1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus for reading out image signals with pixels reduced to one-third or less in a horizontal direction, and signal reading method for the same. The present invention is advantageously applicable to, e.g., a digital camera or an image input apparatus of the type including an image pickup device having high pixel density, e.g., more than several millions of pixels.
2. Description of the Background Art
To implement image quality comparable with one available with a silver halide photo-sensitive type of film, there have been proposed various technologies for increasing the number of pixels of a digital camera that electrically shoots a scene. Japanese patent laid-open publication No. 136391/1998 discloses a solid-state image pickup apparatus constructed to optimize the spatial sampling of an image, to shift pixels with respect to each other in such a manner as to enhance efficient receipt of light, and to reduce moirxc3xa9 and other aliasing signals.
A digital camera of the type including an image pickup device provided with high pixel density is extensively used and directed toward high image quality. It is a common practice with this type of digital camera to effect, before the actual pickup of a still picture, AE/AF (Automatic Exposure/Automatic Focusing) operation and movie drive that causes a scene being picked up to appear on an LCD (Liquid Crystal Display). This, however, brings about a problem that the high pixel density increases the period of time necessary for signal charges resulting from the pickup to be read out and thereby lowers the frame rate. It is to be noted that the high pixel density refers to more than 1,000,000 pixels or so-called megapixels.
To increase the frame rate, signal charges generated in the image pickup device may be read out while being reduced, or thinned, in the vertical direction. Specifically, assume that drive frequency CLK for reading out all of 1,500,000 pixels (1,280xc3x971,024) by progressive scanning is 12.2725 MHz. Then, a single horizontal synchronizing period (1H) and a single vertical synchronizing period (1V) are 1,560 CLK and 1,050 H, respectively. The frame rate is therefore 1/7.5 second. When the signal charges are reduced to one-half in the vertical direction, 1H needs the same period of time while 1V is 525H, resulting in a frame rate of 66.7 milliseconds, i.e., 1/15 second. Even when the signal charges are reduced to one-fourth in the vertical direction, 1V is 262.6H, and therefore the frame rate is as long as 33.4 millisecond or 1/30 second.
Assume that 1,500,000 pixels are read out by progressive scanning and displayed by the movie drive in the conventional image size, i.e., 640xc3x97480. Then, the pixels are reduced to one-half in the horizontal and vertical directions under the above-described conditions. As a result, the number of pixels in the horizontal direction and the number of pixels (lines) in the vertical direction are as great as 640 and 525, respectively. Even the reduction to one-fourth implements only the reduction to one-half in the horizontal direction although reducing the number of pixels in the vertical direction to 262.5, i.e., improving the frame rate. However, because the number of pixels reduced in the vertical direction is short of 480, interpolation must be executed in the vertical direction in order to match the number of pixels to the desired number. On the other hand, in the horizontal direction, all of the 1,280 pixels are read out and then reduced to 640 pixels at the subsequent signal processing stage. It will therefore be seen that strict consideration is not given to the improvement in frame rate in reducing the pixels in the horizontal direction. This is apt to prevent the operator of the camera to miss an adequate actual pickup timing.
It is therefore an object of the present invention to provide a solid-state image pickup apparatus capable of improving the signal output rate during preliminary pickup despite high pixel density, or high image quality, and reading out signals without effecting actual pickup to follow, and a signal reading method for the same.
In accordance with the present invention, a solid-state image pickup apparatus includes an image pickup section and a signal feeding section. The image pickup section includes photosensitive cells arranged bidimensionally and each being shifted from adjoining photosensitive cells in the horizontal and vertical directions for photoelectrically transducing incident light. A color filter has color filter segments each being positioned in front of a particular photosensitive cell in the direction of light incidence for separating colors of incident light representative of a scene. First transfer electrodes each are assigned to a particular photosensitive cell for reading out a signal charge generated by the photosensitive cell. Second transfer electrodes each are assigned to a particular photosensitive cell arranged on at least every third column in the horizontal direction for reading out a signal charge generated by the photosensitive cell. The apparatus sequentially performs preliminary pickup and actual pickup, which reads all of the signal charges out of the photosensitive cells, and executes digital signal processing with the resulting signals. The signal feeding section feeds transfer timing signals for transferring, during preliminary pickup, signal charges output from the photosensitive cells arranged on at least every third column via the second transfer electrodes to vertical transfer paths, vertical drive signals for transferring the signal charges along the vertical transfer paths toward a horizontal transfer path perpendicular to the vertical transfer paths, and horizontal drive signals adjusted in timing for transferring the signal charges along the horizontal transfer path while maintaining the color of the individual signal charge.
Also, in a signal reading method in accordance with the present invention, read out are signal charges generated by photosensitive cells, which are arranged bidimensionally and each is shifted from adjoining photosensitive cells in the horizontal and vertical directions for photoelectrically transducing light of particular separated color incident thereto, in a particular manner for each of preliminary pickup and actual pickup. The method begins with the step of preparing first transfer electrodes each being assigned to a particular photosensitive cell for reading out a signal charge generated by the photosensitive cell. Second transfer electrodes each are assigned to a particular photosensitive cell arranged on at least every third column in the horizontal direction for reading out a signal charge generated by the photosensitive cell. Drive signals are generated for reading out signal charges generated by the photosensitive cells and representative of a shot. During the preliminary pickup, only said second transfer electrodes are rendered conductive by the drive signals, so that pixels are reduced in the horizontal direction. The signal charges read out via the second electrodes are vertically transferred in timed with the drive signals. The vertically transferred signal charges are horizontally transferred in timed with the drive signals adjusted in timing.
A signal reading method applicable to the above image pickup apparatus is also disclosed.